The Androgen Receptor (AR) is an intra-cellular receptor that is a key factor in mediating a wide variety of physiological processes, including regulation of male development, and the behavior of the prostate (see, e.g., Keller, et al., Frontiers in Bioscience, 1:5971, (1996)).
AR is a member of the family of nuclear receptor (NR's), nearly all of which are medically significant (see, e.g., Gronemeyer and Laudet, The Nuclear Receptor Facts Book, Academic Press, (2002)). Nuclear receptors are a superfamily of proteins that specifically bind a physiologically relevant small molecule, such as a hormone. Generally, the binding occurs with high affinity so that apparent Kd's are commonly in the 0.01-20 nM range, depending on the nuclear receptor/ligand pair. The principal action of NR's is to modulate, i.e., enhance or repress, the transcription of DNA. Unlike integral membrane receptors and membrane associated receptors, the nuclear receptors reside in either the cytoplasm or nucleus of eukaryotic cells. As a result of a molecule, such as a hormone, binding to a nuclear receptor, the nuclear receptor changes the ability of a cell to transcribe DNA. Specifically, the nuclear receptors, and in particular AR, regulate gene expression by interacting with specific DNA sequences of target genes (see, e.g., Yamamoto, K., “Steroid receptors regulated transcription of specific genes and gene network,” Ann. Rev. Genetics, 19, 209, (1985); and Beato, M., “Gene regulation by steroid hormones”, Cell, 56:335-344, (1989)).
AR binds hormones, referred to as “androgens”, which include male sex steroids, such as testosterone and 5α-dihydrotestosterone (DHT). The major role of these hormones is the development and maintenance of the male reproductive system and secondary sexual characteristics. In particular, testosterone is responsible for initiating and maintaining spermatogenesis, and the virilization of male internal sex organs, while DHT causes development of external sex organs, as well as secondary sexual characteristics. Androgens also have a variety of anabolic effects, such as increase in mineral bone density, muscle size and strength. Their effects on hair, skin, as well as male behavior are also well known. In normal physiological action, AR plays a role in embryogenesis, homeostasis, the development of sexual organs, reproduction, and cell growth and death in many classes of cells. However, in pathological conditions, AR is implicated in prostate cancers, androgen insensitivity syndromes (AIS), and spinal and bulbar muscular atrophy (Kennedy's disease).
In essence, upon androgen hormone binding, AR binds to DNA, and then acts as a transcription factor that regulates the expression of from about 20 to hundreds of genes depending on the cell type (see, e.g., Keller, E. T., et al., Frontiers in Bioscience, 1: 5971, (1996); and, Beato, M., “Gene regulation by steroid hormones,” Cell, 56: 335-344, (1989)). However, the underlying mechanism is actually more complicated. It is understood that activation of AR, initiated by binding of a hormone such as DHT to the AR ligand binding domain (LBD), changes the three dimensional structure of the LBD, and causes AR to dissociate from chaperones in the cytoplasm and travel into the nucleus where the receptor binds response elements on DNA. This mechanism is effectively a kind of control that ensures that androgen receptors are kept away from DNA molecules until they have been suitably activated.
Androgens have a variety of effects on different tissues in the body. The androgen receptor has wide tissue distribution as can be demonstrated by immunohistochemistry in several tissues e.g., prostate (Zhuang, Y. H., Blauer, M., Pekki, A., et al., “Subcellular location of androgen receptor in rat prostate, seminal vesicle and human osteosarcoma MG-63”, J. Steroid Biochem. and Molec. Biol., 41:693-696, (1992)), skin (see, e.g., Blauer, M., Vaalasti, A., Pauli, S-L., et al., “Location of androgen receptor in human skin”, J. Investigat. Derm., 97:264-268, (1991)), and oral mucosa. The presence of the androgen receptor can also be demonstrated in a diverse range of human tumors, e.g., osteosarcoma (Zhuang, et al., J. Steroid Biochem. and Molec. Biol., 41:693-696, (1992)). In prostatic carcinoma, androgen receptor expression may be of clinical relevance (see, e.g., Demura, T., Kuzumaki, N., Oda, A., et al., “Establishment of monoclonal antibody to human androgen receptor and its clinical application for prostatic cancer”, Am. J. Clinical Oncol., 11(2):S23-S26, (1988)). Mutation of the gene encoding androgen receptor has been reported in prostatic carcinoma (Barrack, E. R., Newmark, J. R., Hardy, D. O., et al., “Androgen receptor gene mutations in human prostate cancer”, J. Cell Biochem., 16D:93, (1992)). Nevertheless, the mechanisms of AR tissue selectivity are only starting to be understood. Development of novel AR ligands that possess tissue specificity would provide new tools for uncovering these mechanisms. Androgen receptor ligands would also have a major therapeutic potential in treating numerous diseases.
Of particular significance, AR has been implicated in the development of prostate cancer and benign prostatic hyperplasia. Prostate cancer is the second leading cause of cancer deaths among men in the United States, and it has a complex etiology (see, e.g., Nelson, K. A., and Witte, J. S., “Androgen Receptor CAG Repeats and Prostate Cancer”, Am. J. Epidemiology, 155:883-890, (2002)). In particular, unregulated AR activity is implicated in metastatic prostate cancers (see, Tenbaum, S., and Baniahmad, A., “Nuclear Hormone Receptors: Structure, Function and Involvement in Disease,” Int. J. Biochem. and Cell Biol., 29:1325-1341, (1997); Taplin, M. E., Shuster, G. J., Frantz, M. E., Spooner, A. E., Ogata, G. K., Keer, H. N., and Balk, S. P., “Mutation of the androgen-receptor gene in metastatic androgen-independent prostate cancer,” New Eng. J. Med., 332:1393-1398, (1995); Gottlieb, B., Beitel, L. K., and Trifiro, M., “Variable Expressivity and Mutation Databases: The Androgen Receptor Gene Mutations Database,” Human Mutation, 17:382-388, (2001)) which are the most common forms of malignancy in men, and androgen insensitivity syndromes (Gottlieb, B., Pinsky, L., Beitel, L. K., and Trifiro, M., “Androgen Insensitivity,” American J. Medical Genetics (Semin. Med. Genet.), 89, 210-217, (1999)), but its role is not yet fully understood. Consequently, current research in prostate cancer is aimed at finding new ways to inhibit AR function in pathological states.
Currently, non-steroidal AR antagonists are used clinically to treat early stages of prostate cancer (see, e.g., McLeod, D. G., “Antiandrogenic Drugs,” Cancer, 71(3), 1046-1049, (1993)). However, these agents still cause undesirable side effects, such as breast tenderness and gynecomastia (see, e.g., See, W. A., et al., “Bicalutamide as Immediate Therapy Either Alone or as Adjuvant to Standard Care of Patients with Localized or Locally Advanced Prostate Cancer: First Analysis of the Early Prostate Cancer Program,” The Journal of Urology, 168, 429-435, (2002)). Moreover, their effect ceases as the prostate cancer progresses to androgen-independent stages.
While there is considerable interest in developing selective AR modulators, the number of ligands developed thus far is still limited. Current treatment of prostate cancer is often with anti-testosterones, such as flutamide (Eulexin, or cyproterone acetate), nilutamide, and bicalutamide (casodex), which suppress AR function. Flutamide and bicalutamide, whose structures are shown in FIG. 1, were the earliest non-steroidal AR antagonists developed and are aniline derivatives containing strong electron-withdrawing substituents such as nitro or cyano (see, e.g., Tucker, H., Crook, J. W., and Chesterson, G. J., “Nonsteroidal antiandrogens: Synthesis and structure-activity relationships of 3-substituted derivatives of 2-hydroxypropionanilides”, Journal of Medicinal Chemistry, 31(5), 954-959, (1988)). However, after 3-5 years of treatment with these agents, the treatment becomes less effective. In particular, prostate-specific antigen (PSA) levels are seen to rise in patients; the presence of such antigens indicates AR activation. The rise in malignant transcriptional activity has been attributed to AR being activated inappropriately.
Recently, several other scaffolds for non-steroidal AR ligands have been reported. Several sets of ligands that combine structural features of bicalutamide and flutamide with [2.2.1]-bicycloazahydantoins (structure 1 in FIG. 1) (Balog, A., et al., “The synthesis and evaluation of [2.2.1]-bicycloazahydantoins as androgen receptor antagonists,” Bioorganic and Medicinal Chemistry Letters, 14, 6107-6711, (2004)) or bicyclic 1H-isoindole-1,3(2H)-dione (structure 4 in FIG. 1) (Salvati, M. E., et al., “Identification of a novel class of androgen receptor antagonists based on the bicyclic-1H-isoindole-1,3(2H)-dione nucleus,” Bioorganic and Medicinal Chemistry Letters, 15, 389-393, (2005); Salvati, M. E., et al., “Structure based approach to the design of bicyclic-1H-isoindole-1,3(2H)-dione based androgen receptor antagonists,” Bioorganic and Medicinal Chemistry Letters, 15, 271-276, (2005)) scaffolds have been developed.
A number of ligands based on 4-trifluoromethyl-2-quinolone scaffold, for example structures 3 and 4 (Hamann, L. G., et al., “Synthesis and Biological Activity of a Novel Series of Nonsteroidal, Peripherally Selective Androgen Receptor Antagonists Derived from 1,2-Dihydropyridono[5,6-g]quinolines,” Journal of Medicinal Chemistry, 41(4), 623-639, (1998); Konga, J. W., et al., “Effects of isosteric pyridone replacements in androgen receptor antagonists based on 1,2-dihydro- and 1,2,3,4-tetrahydro-2,2-dimethyl-6-trifluoromethyl-8-pyridono[5,6-g]quinolines,” Bioorganic and Medicinal Chemistry Letters, 10, 411-414, (2000)) have also been developed. Varying the substitution pattern on the outer ring afforded a series of ligands with a spectrum of effects, from full agonism to full antagonism (Hamann, L. G., et al., “Discovery of a Potent, Orally Active, Nonsteroidal Androgen Receptor Agonist: 4-Ethyl-1,2,3,4-tetrahydro-6-(trifluoromethyl)-8-pyridono[5,6-g]-quinoline (LG121071)”, Journal of Medicinal Chemistry, 42(2), 210-212, (1999); Zhi, L., et al., “Switching Androgen Receptor Antagonists to Agonists by Modifying C-Ring Substituents on Piperidino[3,2-g]quinolone,” Bioorganic and Medicinal Chemistry Letters, 9, 1009-1012, (1999)).
Substituted phthalimides (structure 5) have also been shown to be efficient AR ligands (Miyachi, H., et al., “Potent novel nonsteroidal androgen antagonists with a phthalimide skeleton”, Bioorganic and Medicinal Chemistry Letters, 7(11), 1483-1488, (1997)). Recently, ligands based on the iso-oxazolidinone scaffold (structure 6) have been reported to be up to 200 times more potent than flutamide (Ishioka, T., et al., “Novel Non-Steroidal/Non-Aniline Type Androgen Antagonists with an Isoxazolone Moiety,” Bioorganic and Medicinal Chemistry, 10(5), 1555-1566, (2002); Ishioka, T., et al., “Anti-Androgens with full antagonistic activity toward human prostate tumor LNCaP cells with mutated androgen receptor” Bioorganic and Medicinal Chemistry Letters, 13(16), 2655-2658, (2003)). Moreover, unlike flutamide, these ligands act as antagonist even on the mutated AR (for example, in LNCaP cells) (Ishioka, T., et al., Bioorg. and Med. Chem. Lett., 13(16), 2655-2658, (2003)).
The other principal therapeutic application of androgen receptor ligands is to male hormone replacement therapy. The level of testosterone decreases significantly in older men, thus resulting in osteopenia and loss of lean body mass. Nevertheless, the use of endogenous androgens and steroidal AR agonists in male hormone replacement therapy is limited due to a variety of undesirable side effects (see, e.g., Zhi, L., and E. Martinborough, “Selective Androgen Receptor Modulators (SARMs),” Annual Reports in Medicinal Chemistry, 36, 169-180, (2001)). Thus, development of a selective AR modulator with androgen effect in bones and muscles, but not in the prostate, would be highly desirable. Tissue-selective AR modulators can also be used for treatment of reproductive disorders and male hypogonadism (see, e.g., Zhi, L., Ann. Repts. in Med. Chem., 36, 169-180, (2001)).
Thus, compounds that exhibit tissue selective antagonism for the androgen receptor remain desirable and have yet to be satisfactorily developed.
The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known, or part of the common general knowledge as at the priority date of any of the claims.
Throughout the description and claims of the specification the word “comprise” and variations thereof, such as “comprising” and “comprises”, is not intended to exclude other additives, components, integers or steps.